As methods of seawater desalination, a method called “evaporation method” which obtains fresh water by distilling sea water, and a membrane method (herein referred to as “RO membrane method”) which filters seawater under high pressure by using a reverse osmosis membrane (hereinafter referred to as “RO membrane”) have been put to practical use. In the RO membrane method, seawater, pressure of which is increased by a high-pressure RO pump, is passed through an RO membrane, and thereby the seawater is desalinated. The RO membrane method generally requires lower desalination cost than that of the evaporation method. Therefore, the RO membrane method has come into wide use in place of the evaporation method (for example, see Jpn. Pat. Appln. KOKAI Publication No. 08-206460).
In the meantime, in seawater desalination systems using the RO membrane method, seawater is subjected to pretreatment before being desalinated through an RO membrane, to remove suspended solids in the obtained seawater. In recent years, a micro filter membrane (MF membrane) or an ultrafiltration membrane (UF membrane) is used for the pretreatment (for example, see Jpn. Pat. Appln. KOKAI Publication No. 2009-172462 and No. 2008-100219). Seawater which has been subjected to pretreatment is temporarily held in a control vessel. When a required water feed quantity to the RO membrane is increased or pretreatment quantity is reduced, the control vessel stably supplies the pretreated seawater to the high-pressure RO pump. The high-pressure RO pump increases the pressure of seawater supplied from the control vessel, and feeds the seawater to the RO membrane. In the RO membrane method, the pretreated seawater must be temporarily held in the control vessel, and thus there is the fear that suspended solids such as microorganisms and dirt may be mixed into the seawater, and the water quality may deteriorate. In addition, such a structure including the control vessel increases the size of the seawater desalination system.